A read channel integrated circuit (IC) is one of the core electronic components in a modern hard disk drive. For example, in a magnetic recording system, a read channel converts and encodes data to enable the magnetic recording heads to write data to the disk drive and then read back the data accurately. The disks in a drive have many tracks on them and each track consists of mostly user or “read” data sectors as well as “servo” data sectors embedded between the read sectors. The information recorded in the servo sectors helps to position the magnetic recording head on track so that the information stored in the read sectors is retrieved properly.
When a disk drive is powered up, the magnetic recording head will try to locate a servo sector, during a servo search mode. In particular, the magnetic recording head will try to locate a servo preamble (i.e., a preamble pattern written at the servo frequency), that allows the recording system to recover the timing and gain of the written servo data. Once the search mode identifies a servo sector, the signal timing and gain information can be obtained from the known servo preamble pattern and the various servo data fields can be detected. When a disk drive is initialized, however, the magnetic head could reside over any portion of a read field or a servo field. Thus, finding the closest servo preamble is a challenging task.
The servo and read sectors typically both begin with the same known 2T preamble pattern. The read preamble is followed by a read address mark and encoded user data. The servo preamble is followed by a servo address mark and various servo data. The goal of the search mode is to first find the servo preamble field and then initiate “normal” servo operations. Normal mode servo operations will be successful only if the search mode properly finds the servo preamble field. If the search mode confuses a read preamble field for a servo preamble field, the normal mode of servo operations will not be successful, since there is no servo information following the read preamble. When normal mode operations improperly begin on a read preamble field, the servo address mark detection logic will continue looking for the address mark indefinitely or a servo address mark will be falsely identified in the user data field, thereby misdirecting the servo control circuitry.
A need therefore exists for a method and apparatus for restarting the search mode as soon as possible to start a new search for the servo preamble field, in the event that a read preamble is mistaken for a servo preamble.